Device for controlling the flow of electrons in a betatron



Dec. 4, 1956 K. GUND ET AL 2,773,183

DEVICE FOR CONTROLLING THE FLOW OF ELECTRONS IN A BETATRON Filed Oct. 28, 1950 CugRE/VT Sal/26E jawed OF 056/; n no:

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United States Patent DEVICE FOR CONTROLLING THE FLOW OF ELECTRONS IN A BETATRON Konrad Gun d, Hans Berger, and Kurt von Sanden, Erlangen, Germany Application October 28, 1950, Serial No. 192,732

Claims priority, application Germany October 31, 1949 5 Claims. (Cl. 250--27) This invention is concerned with a device for controlling the flow of electrons in a betatron in which the elec trons are injected into the acceleration chamber of the betatron within the stabilizing region of the guiding field approximately azimuthally in an orbital path and furnishes means for increasing the space charge Within the acceleration chamber, and therewith means for increasing the yield of electrons and the efficiency of operation.

The obtainable space charge within the accelerating space increases with the increase of the stabilizing forces in the guiding field which at the instant of the electron injection counteract the mutual repulsion of the electrons, and for producing a high space charge in the accelerating space it is accordingly known to inject the electrons into the accelerating space with as great an initial velocity as possible at a time when the guiding field has reached a strength which corresponds to the injection velocity of the electrons.

As compared with this known procedure, the invention proposes to increase the space charge in the accelerating space by uniformly distributing'the electrons during the injection in a spiral path over the accelerating space thus permitting utilization of the entire width thereof for the filling with electrons to the greatest possible capacity of the space.

In accordance with the invention, this is done by means which produce a disturbance of the 1:2 condition which defines the mutual relationship that must obtain at any instant of the electron acceleration between the magnetic main field causing acceleration and the magnetic guide field, so as to secure the electrons during the acceleration relative to the acceleration orbit. The disturbance of this 1:2 condition is produced so that, for theduration of the injection of the electrons, if they are injected into the inner zone of the stabilizing guiding field region, the guiding field increases less than proportional relative to the acceleration field; and so that, if the electrons are injected into the outer zone of the stabilizing guiding field, the accelerating field increases less than proportional relative to the guiding field.

This disturbance of the 1:2 condition during the injection of the electrons causes, in the case of injection from the inside a much more rapid expansion and, in the case of injection from the outside, a considerably more rapid contraction of the electron paths. It may be obtained, e. g., by the use of a coil which surrounds only the accelerating field and causing a current variable as a function of time to fiow through this coil immediately preceding the beginning of the injection and for the duration thereof. The strength and direction of such current is such that the entire magnetic field within this coil is varied as a function of time so that as a reaction thereto the guiding field increases either more rapidly or more slowly, due to the fact that the total field of the betatron must have a sinusoidal rise because it is supplied by an electric oscillating circuit. If the coil is disposed at the outer border of the accelerating field, i. e., close to the inner border of the guiding field, the magnetic aux- 2,773,183 Patented Dec. 4, 1956 iliary field of the coil will cause weakening of one and at the same time strengthening of the other field. The auxiliary field produced by the coil may be given any desired strength and operative duration by suitable choice of the number of ampere windings of the coil and the curve of the supply current therefor. it is particularly advantageous to discharge a capacitor through the coil by means of a thyratron. The self inductance of the coil produces a sinusoidal discharge current and the thyratron interrupts this current at the moment of its zero passage. This discharge current, which is in the form of a sine half-wave, produces a corresponding magnetic auxiliary field of a predetermined field strength which is superimposed to subtract from the guiding field of the betatron and to add to the other or accelerating field, as shown schematically in Fig. l, and which at suitably chosen capacitor voltage just compensates for a brief interval the increase of the one field of the betatron, e. g., in the case of injection from the inside, the guiding field, holding said field at a constant value most favorable for securing the electrons. The result is that all electrons are secured under identical conditions and are uniformly distributed in the accelerating space upon a spiral path, due to the great increase of the accelerating field.

The control of the thyratron may be obtained, if desired, by the use of an impulse transformer, from the exciting circuit of the betatron in such a manner that the ignition of the thyratron occurs shortly after zero passage of the betatron field. A phase shift device may be employed for delaying the igniting of the thyratron relative to the zero passage of the accelerating field so as to choose the field strength at which, for example, one of the fields is to be held constant.

The invention and its various objects and features will now be described with reference to the accompanying drawing showing an embodiment thereof. In the drawing,

Fig. 1 shows in diagrammatic manner the magnetic yoke of a betatron with a so-called securing coil disposed in the air gap between the accelerating poles;

Fig. 2 indicates an embodiment of a circuit for the securing coil; and

Figs. 3a and 3b respectively illustrate the characteristics of the current I in the securing coil plotted against time t (Fig. 3a) and the current I producing the accelerating and guiding fluxes, plotted against time t; the electrons being assumed to be injected closely within the outer borderof the guiding field.

The vacuum vessel in which the electrons are accelerated and which nearly fills the space between the guiding field poles, is indicated in Fig. 1 by numeral 20.

On the yoke 1, in the embodiment shown in Fig. 1, are secured the oppositely disposed poles 2 and 3 whose central cylindrical portions (accelerating poles) carry the accelerating flux and whose ring-shaped. or annular portions (guiding field poles), which surround the central portions, carry the guiding flux. Both fluxes are generated by the cyclic alternating current of a source, e. g., a line frequency which flows in the exciter winding 4. Within the air gap between the acceleration poles, close to the inner border of the guiding field poles, is disposed a coil 5 which serves for maintaining constant one of the betatron fields during the injection of the electrons and which is therefore designated as securing coil.

As may be seen from Fig. 2, the coil 5 is connected by means of a thyratron 6 with a capacitor 7 which is charged through a resistor 8 from a current source connected to the terminals 9. The discharge current, which flows through the coil 5 during the time when the tube is conductive, has a sinusoidal shape due to the self induction of the coil. The thyratron 6 is controlled from the exciter oscillating circuit 10 of the betatron by the use of a phase shift device 11 and an impulse transformer 12, in such a manner that it ignites shortly after the exciter current has passed through zero. After ignition the discharge current of the capacitor 7 maintains the discharge through the thyratron up to the moment when it passes through zero, that is to say, only during its positive half-wave.

The current impulse in the coil therefore has the shape shown in Fig. 3a at 13 and generates an auxiliary magnetic field which has, as shown in the right half of Fig. 1 by magnetic lines of force provided with arrows, partly the same direction as the normal betatron flux shown in the left half, and partly the opposite direction. In the case illustrated in Fig. 1, the additional magnetic flux in the coil 5 causes the accelerating flux 14 (Fig. 3b) to remain constant for a short intervalduring the interval t1while the guiding field flux 15 increases rapidly during this interval. If the electrons are during this interval-which is, e. g., 3 10* sees, the betatron being assumed to be supplied with ,SO-cycle alternating current at a duration of the current impulse in the coil of 1() secs.injected closely within the outer borders of the guiding field, they will rapidly drift toward the inside.

Co-pending applications Ser. Nos. 192,731, now Patent Number 2,721,949 issued Oct. 25, 1955, 192,733, now Patent Number 2,665,392 issued Jan. 5, 1954, and 226,729 now abandoned, all filed October 28, 1950, may be consulted for structural details which may be considered pertinent to the invention.

In this application, the term betatron refers to a device in which electrons are moved along approximately circular paths in a magnetic guiding field, and accelerated by means of a magnetic flux of increasing strength passing through said circular paths.

We claim:

1. In a betatron having field poles and an exciter coil therefor which is excited .in an alternating current circuit for producing respectively a magnetic guiding field and a magnetic accelerating field, said fields increasing in operation in accordance with the 1:2 condition, and having a vacuum vessel forming an accelerating chamber in which electrons are to be accelerated along a stabilized orbital path, said electrons being injected into said accelerating space from an electron source disposed outside of said stabilized orbital path; a device for disturbing said 1:2 condition to trap an increased number of electrons injected for acceleration so as to increase the yield of accelerated electrons, said device comprising an auxiliary coil completely surrounded circumferentially by said vessel, a capacitor, a thyratron, means for connecting said auxiliary coil and said capacitor and said thyratron in a resonance circuit, means controlled by said exciter circuit shortly after the instant of Zero passage of the current therein and coincident with the instant of injection of electrons from said electron source for causing said thyratron to become momentarily conductive for the purpose of producing at said instant of electron injection in said auxiliary coil a current impulse which decays substantially sinusoidally, said impulse causing said disturbance of said 1:2 condition, the characteristics of said resonance circuit and the direction of current flow of said impulse through said auxiliary coil causing distribution of the injected electrons along a spiral path substantially throughout the entirety of said acceleration chamber to increase the density of injected and trapped electrons at the instant of injection thereof.

2. A betatron wherein electrons are injected from said electron source at a point inside the stabilized orbital path and having a device according to claim 1, wherein the direction of current flow of said impulse through said auxiliary coil is such as to cause the guide field flux to increase during the electron injection less than proportional with the accelerating field.

3. A betatron wherein electrons are injected from said electron source at a point outside the stabilized orbital path and having a device according to claim 1, wherein the direction of current flow of said impulse through said auxiliary coil is such as to cause the acceleration flux to increase less than proportional with the guide field flux.

4. A betatron wherein electrons are injected from said electron source at a point disposed within the inner zone of said accelerating chamber and having a device according to claim 1, wherein the guiding field is responsive to said impulse maintained constant for the duration of electron injection at a value which is most favorable for the trapping of increased number of electrons.

5. A betatron having a device according to claim 1, wherein said auxiliary coil is disposed peripherally of said acceleration field.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,305 Kerst Sept. 29, 1942 2,331,788 Baldwin Oct. 12, 1943 2,480,169 Westendorp Aug. 30, 1949 2,528,525 Langmuir Nov. 7, 1950 2,528,526 Westendorp Nov. 7, 1950 2,533,859 Wideroe ,Dec. 12, .1950 2,540,853 Wilkinson Feb. 6, 1951 2,654,838 Wideroe Oct. 6, 1953 

